1. Field of the Invention
The present invention relates to a power conversion system, and in particular relates to a power resonant conversion system with over-current protection.
2. Description of the Related Art
The current trends in DC-DC converters, like most power products, are towards higher efficiencies, higher power densities, higher reliabilities and lower costs. In DC-DC converters, resonant converters (such as LLC converter) have the advantage of high efficiency in wide input voltage ranges. Therefore, there are more and more applications using resonant converters in recent years.
However, in practical applications, over-current protection (OCP) of resonant converters is an important issue. In an over-loaded state or short-circuit state, the resonant current of a resonant converters becomes extremely large. The resonant converter could be damaged due to over-current, if without restrictions and protections. FIG. 1 is a diagram of a resonant converter. At startup, the output capacitor CL in a secondary winding can be folded to the primary winding to parallel connect the LM. When the resonant converter is powered on, there is no voltage across the output capacitor CL. That is, the voltage levels on the two plates of the output capacitor are the same. Thus, the output capacitor CL can be regarded as a conducting wire. Therefore, the resonant converter can equivalently comprise a high frequency input voltage across the two terminals of the resonant tank/circuit, such that the impulsion of the resonant current is larger. The impulsion of the output current (the impulsion of the current flowing into the output capacitor CL) is also larger. Over-current protection not only provides rapid protection when the resonant conversion system is over-loaded and short-circuited, but also restricts the impulsion of the current at startup. In order to solve this problem, some methods have been developed to restrict current. However, there are some defects in these methods.
One conventional method for protecting a resonant converter with over-current protection is to increase the switching frequency. In the method, the switching frequency is increased much more than the resonant frequency of the resonant converter; thereby, increasing the resistance of the resonant tank/circuit for performing the current restriction. Although operation is easy, there are some defects in the method. For example, it results in increasing power consumption of the switch and the heat dissipation requirement becomes more increased, because the switching frequency of the resonant tank/circuit is higher than the normal operating frequency in an over-current protection state.
A second conventional method for protecting a resonant converter with over-current protection is to use a Pulse Width Modulation (PWM). In the method, the switching frequency of the resonant tank/circuit is increased at first. When the switching frequency is increased over a specific level, the PWM circuit starts to decrease the voltage applied on the resonant tank/circuit in order to perform the current restriction. However, the method is difficult to the control. During the current restriction procedure, the switching devices cannot be operated under the zero voltage switching ZVS, wherein the power consumption thereof would be increased.
In a third conventional method for protecting a resonant converter to perform current restriction, a diode is used to clamp the voltage across the resonant capacitor on the input voltage. FIG. 2 shows a symmetric full bridge resonant converter of the method. However, the clamping voltage is a DC input voltage Vin, and thus, the maximum voltage of the resonant capacitor Cr is only equal to the input voltage Vin and the change of the resonant voltage depends on the input voltage Vin. Thus, the result is more restrictions on the resonant circuit design.